Composite material structure

ABSTRACT

A laminated composite material structure, in the form of a load-bearing rib of a wing of an aircraft, comprises an upper laminated portion, which is angled with respect to a middle laminated portion, and a curved corner portion being continuous with and interposed between the upper and middle portions. (The rib has a curve that connects the upper and middle portions, which are perpendicular or transverse to each other). The corner portion is in the form of a kinked portion or joggled, that is, the portion includes two regions of positive curvature (concave regions) and a region of negative curvature (convex region) interposed therebetween, whereby the formation or propagation of delaminations or cracks at the bend between the middle and upper portions, due to fuel pressure loading and low through-thickness strength, may be reduced.

This application is the US national phase of international applicationPCT/GB02/04046 filed in English 6 Sep. 2002, which designated the US.PCT/GB02/04046 claims priority to GB Application No. 0122050.8, filed 13Sep. 2001. The entire contents of these applications are incorporatedherein by reference.

This invention relates to a laminated composite material structure forforming a part of an aircraft, and an aircraft wing structure oraircraft including such a laminated composite material structure.

The use of laminated composite materials in the aerospace industry iswell known. Composite materials have been successfully used in wingskins of aircraft for example. It has been proposed that laminatedcomposite materials could be used in components having a morecomplicated shape, for example a rib of a wing of a passenger carryingaircraft. It is common for such components to be subjected to variousloads in use and for such components to have angled portions (portionsthat extend away from one another at an angle, commonly a right angle).

It has been found during trials that there are various problemsassociated with using such laminated composite materials in load bearingcomponents having angled portions. In particular, laminated materialscommonly suffer from having low through-thickness strength, that is, thematerial may be prone to failure when exposed to (relatively low)tensile forces in a direction normal to the layers of the material. Suchforces are not necessarily applied directly to the material, but may begenerated within the material as a reaction to external forces that areapplied.

FIG. 1 illustrates an example of the problems that may arise when usinglaminated composite materials having low through-thickness strength.FIG. 1 shows schematically, in cross-section, a section of a wingincluding a generally C-shaped rib 1, made of laminated compositematerial, attached to an upper wing skin 3 a and to a lower wing skin 3b. Fuel 2 stored in the space defined between the rib 1 and the wingskins 3 a, 3 b exerts forces, schematically represented by arrows 2 a,on the rib 1 (for example, because the fuel occupies only part of theavailable space and is thus able to move inside the wing structure andcollide with the rib 1 or because the fuel is stored, possiblyaccidentally, under pressure). The rib 1 includes two angled portions 7a, 7 b, where portions of the rib 1 curve through 90 degrees. In use,the forces exerted on the rib 1 are such that the angled portions 7 a, 7b are urged to open out to a greater angle, thereby urging the inner andouter surfaces of the angled portions away from each other. Such forcescan cause the layers of the laminated composite material to delaminateand/or cracks to form in the regions of the angled portions 7 a, 7 b(FIG. 1 shows such cracks/delaminations schematically as faults 4 a, 4b). Once such cracks and/or delaminations are formed they can quicklypropagate through the structure.

It is an object of the invention to provide a laminated compositematerial structure for forming a part of an aircraft, wherein thestructure has an angled portion with improved resistance to delaminationand/or cracking.

According to the invention there is provided a laminatedcomposite-material structure for forming a part of an aircraft, thestructure comprising first and second laminated portions angled withrespect to each other and a third laminated portion being continuouswith and interposed between the first and second laminated portions,wherein the third laminated portion includes a region of positivecurvature and a region of negative curvature.

By introducing regions of both positive and negative curvature in thethird portion, the flexibility of the third portion may be improved incomparison to the case where the first and second portions are separatedonly by a portion having a constant radius of curvature. Such increasedflexibility reduces the likelihood of delamination and/or cracks fromforming.

Thus, the provision of the regions of both positive and negativecurvature in the third portion is advantageously for the purpose ofincreasing the flexibility of the third portion, whereby the risk ofdelamination may be reduced.

The arrangement of the invention may also assist in arresting orlimiting the propagation of cracks and/or delaminations. Such faultspropagate more easily when the layers of the composite material are intension. By having regions of opposite curvature, at least one region ofthe third portion may be under much less tension than would otherwise bethe case. Such a region may even be in compression through the layers.Such compressive forces may assist greatly in arresting or limitingcrack propagation and arresting or limiting delamination.

Furthermore, the present invention may assist in the manufacture ofcomponents incorporating the structure of the invention. It is knownthat, when manufacturing laminated composite materials having curvedsurfaces, after the fibre layers of the material have been laid and thematerial has been set in resin, there is a certain amount of shrinkageduring processing. This shrinkage can cause curved surfaces to“spring-in”, that is, the curvature of the surface has a tendency toincrease (i.e. the radius of curvature decreases). Whilst it is possibleto predict, to a limited extent, the amount of spring-in a givencomponent may undergo, it would of course be desirable to reduce theeffect. Having, regions of both positive and negative curvature in thethird portion may therefore be of further advantage in that it may limitthe effect of “spring-in” after manufacture of a given component.

In most cases, the regions of positive and negative curvature will bepositioned one after the other in the direction from the first andsecond laminated portions. The regions of positive and negativecurvature are preferably arranged one directly after the other withlittle or no regions of zero (or near zero) curvature therebetween.

Of course, the significance of there being both a region of positivecurvature and a region of negative curvature is that there is a firstregion of curvature of one sign (either positive or negative) and asecond region of curvature, which is opposite in sign to the first, sothat if the first region of curvature is positive, the second region ofcurvature is negative, and vice versa.

It will be understood that the regions of positive curvature andnegative curvature may be considered as comprising a concave region anda convex region.

At least one of the regions of curvature may have a cross-section in thegeneral form of an arc having a substantially constant radius ofcurvature. It will be understood that, depending on the thickness of thelaminated portions, the radius of curvature on one side of the structuremay be significantly different from the radius of curvature on theopposite side of the structure. In such a case, the radius of curvatureof the arc may, for the sake of convenience, be taken as the radius ofcurvature of the middle of the structure. The regions of constantcurvature may of course be interposed between regions of zero and/orvarying curvature. The substantially constant radius of curvature may beof a size that is the same (within a factor of 10) as the averagethickness of the first and second laminated portions.

The third portion preferably includes a first region of curvaturepositioned between second and third regions of curvature of the oppositesign to that of the first region of curvature. The regions of curvatureare advantageously positioned one after the other in the directionbetween the first and second laminated portions. The first regionpreferably has a curvature having a magnitude that is not greater thanthat of either the curvatures of the second and third regions ofcurvature. For example, the first region has a radius of curvature thatis not greater than the radius of curvature of either of the radii ofcurvature of the second and third regions of curvature. Preferably, theaverage radii of curvature of the regions of curvature of the thirdportion are within a factor of 4, more preferably a factor of 2, of eachother. The radii of curvature of the second and third regions ofcurvature may be substantially equal. The third portion may have one ormore corrugations and may be generally corrugated in shape.

The first and second laminated portions may be curved in shape (forexample, the first and second laminated portions may have a generallysinusoidal shape). Generally, but not necessarily, the first and secondlaminated portions are conveniently substantially planar. It should benoted however that the first and second laminated portions may have agently curving surface whilst still being considered as beingsubstantially planar.

The first and second laminated portions may be at an angle of between60° and 120° to each other and may for example be transverse to eachother.

Preferably, the first, second and third portions have a substantiallyconstant cross-section, the cross-section including all of the first,second and third portions.

The structure may form at least a part of a rib for a wing of anaircraft. In the embodiment described below the structure is a rib of awing of an aircraft. In the case where the structure forms a rib, or apart thereof, the first portion of the structure may form at least apart of the portion of the rib which attaches the wing skin to the rib.Such a rib may include two integrally formed parts, each part being of aconstruction in accordance with the structure of the present invention,the first portions of each structure forming the respective portions ofthe rib that enable the upper and lower wing skins, respectively, to beattached to the rib.

The structure may be used to advantage in other parts of an aircraft andthe structure may for example form at least a part of any of thefollowing aircraft components or systems: a spar, a spar for a wing, thesub-structure of the wing box, an aileron, a flap, a spoiler, atail-plane, a part of the fuselage frame, or the fuselage.

The invention further provides an aircraft wing structure including astructure of the invention described herein, for example, including arib incorporating the structure of the invention. The invention yetfurther provides an aircraft including a structure of the invention asdescribed herein, for example, including a wing structure as describedimmediately above.

The invention is described further, by way of example, with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic view of a section of a wing structure illustratingthe problems that the invention seeks to mitigate,

FIG. 2 is a schematic (and not to scale) view of a section of a wingstructure having a rib including two joggles,

FIG. 3 is a schematic perspective view of the rib shown in FIG. 2, and

FIG. 4 is an aircraft incorporating the rib of FIG. 2.

A brief description of the rib 1 shown in FIG. 1 is given above.

FIG. 2 shows a part of a wing structure which includes rib 1 bolted toupper and lower wing skins 3 a, 3 b (shown in part only in the Figures),the rib being made from laminated composite material. The cross-sectionof the rib 1 in the region of the section illustrated in FIG. 2 issubstantially constant and is generally C-shaped. Fuel 2 is stored inthe space defined between the rib 1 and the wing skins 3 a, 3 b. In use,the fuel exerts forces, schematically represented by arrows 2 a, on therib 1. The rib 1 has: a generally planar upper portion 1 a whichfacilitates connection to the upper wing skin 3 a, by means of bolts 6a; a generally planar lower portion 1 b which facilitates connection tothe lower wing skin 3 b, by means of bolts 6 b, the upper and lowerportions being generally parallel to and spaced apart from each other;and a middle generally planar portion 1 c interposed between andconnecting the upper and lower portions 1 a, 1 b, via corner portions ofthe rib 1, the middle portion 1 c being generally perpendicular to eachof the upper and lower portions 1 a, 1 b. It will be understood that theupper, middle and lower portions 1 a, 1 b, 1 c of the rib 1 are formedas a monolithic structure.

The upper and lower portions 1 a, 1 b of the rib shown in FIG. 1 areeach connected to the middle portion 1 c by means of a simple curvedportion (or bend) 7 a, 7 b. Each curved portion 7 a, 7 b has asubstantially constant radius of curvature. The rib 1 of FIG. 2 differsfrom that shown in FIG. 1 in that the respective portions of the rib 1interposed between the upper portion 1 a and the middle portion 1 c andbetween the lower portion 1 b and the middle portion 1 c are in the formof joggles 5 a, 5 b. It will be understood that in the context of thepresent invention a joggle may be in the form of a kink, corrugation,triple-bend (or higher order bend), or the like in the rib 1. Eachjoggle 5 a, 5 b is of course integrally formed with the portions of therib 1 that it joins.

The joggles 5 a, 5 b shown in FIG. 2 comprise two regions 11 of positivecurvature (concave regions, when viewed from the inside of the C-shapedrib) and a single region 12 of negative curvature (a convex region, whenviewed from the inside of the C-shaped rib) positioned therebetween. (Ofcourse, depending on one's viewpoint, the joggle could be considered ascomprising a single region of positive curvature disposed between tworegions of negative curvature.) The radii of curvature of the regions 11of positive curvature are each 13 mm. The radius of curvature of theregion 12 of negative curvature is 9 mm. By way of comparison, thethickness of the rib in the region of the joggles is about 4 mm. The rib1 described with reference to the drawings is in the wing in a regionclose to the fuselage and is therefore relatively thick. The ribs nearerthe tips of the wings might, for example, may have a thickness of about5 mm or less.

The shape of each joggle 5 a, 5 b is such that the shortest distancealong the surface of the joggle, between the notional boundaries 21between the portions to either side of the joggle, is longer than wouldotherwise be the case (compare with the corner joints 7 a, 7 b of therib 1 of FIG. 1, for example). Thus, if the rib 1 is extended by a givendistance the mechanical strain suffered by the joggle 5 a, 5 b may berelatively less, and thus the stresses within the corner portion may berelatively less.

Moreover, the joggles 5 a, 5 b each have a shape that generally curvesand weaves, the shape not being extremely convoluted. Thus, withreference to the upper joggle 5 a of FIG. 2, the shape does not deviategreatly from the notional path directly linking the notional boundaries21 between the joggle and the portions 1 a, 1 c either side thereof.Thus, the shape is such that the direction at any point along thenotional route described by the joggle 5 a moving from one boundary 21to the other boundary 21 includes a positive component in the generaldirection from the one boundary to the other.

FIG. 3 shows the general shape of the rib 1 (the joggles being omittedfor the sake of clarity). As can be seen in FIG. 3, the cross-sectionis, in several regions, substantially constant. The rib 1 does not,however, have a constant cross-sectional shape throughout its length.For example, the rib 1 includes a cut-out 8, of circular cross-section,in the middle portion 1 c. In this example the cut-out 8 is provided toallow the passage of internal fuel pipes (not shown). There are manydesign reasons why the cut-out 8 may be present, including to allow thepassage of fuel pipes, such as for lightening the structure or for fueltransfer. Also, there are cut-outs 9 in the region of the boundarybetween the middle portion 1 c and the upper portion 1 a and in theregion of the boundary between the middle portion 1 c and the lowerportion 1 b, those cut-outs 9 being provided to allow stringers (notshown in FIG. 3), also know as stiffeners, to pass through the rib 1.Thus the joggles are not continuous along the length of the rib 1. Therib 1 also includes vertical rib stiffeners 10 provided to improve theload bearing capability of the rib 1.

The composite fibre and matrix material, which forms the rib 1,comprises a series of plies of carbon fibre set in epoxy resin material(although the fibres could be set in a thermoplastic matrix). With agiven ply, all the fibres run in the same direction: in some plies thatis the vertical direction (referred to hereinafter as 0°) as shown inFIG. 3; in other plies it is plus or minus 45° to that direction and instill other plies it is at 90° to that direction. Of course it should beunderstood that these ply directions are simply examples of a typicalmaterial and that many other directions of fibres may be employed.

As the rib 1 is required to withstand complex loading including bothbending and shear loads, the lay-up of fibres will include a combinationof plies in different directions. Different lay-ups are used for thedifferent regions of the rib. For example, the middle portion 1 c(commonly referred to as the web of the rib) may have lay-ups rangingfrom 10/80/10 (i.e. 10% in the 0° direction, 80% in the ±45° directionand 10% in the 90° direction) for ribs that are mostly exposed to shearloads to 30/40/30 for ribs that are exposed to greater bending loads. Inthe present embodiment the lay-up of the rib 1 in the middle portion 1 cis 25/50/25 (i.e. quasi-isotropic). The upper and lower portions of therib 1 a, 1 b (commonly referred to as the rib feet) may have a lay-upvarying from 25/50/25 (quasi-isotropic) to 50/40/10 (rib feet bending).In the present embodiment the lay-up of the rib 1 in the upper and lowerportions 1 a, 1 b is 25/50/25. Thus there is no variation in the lay-upacross the rib in the region of the joggle. (Whilst the rib may havediffering lay-ups in different regions it is preferred that the lay-upin the region of the joggle is substantially the same.) The rib 1 isformed of sixteen layers of fibre material (only five layers are shownin FIG. 2 for the sake of clarity). The rib 1 may be formed in anappropriately shaped mould in accordance with known manufacturingtechniques.

The inclusion of joggles 5 a, 5 b in the rib 1 shown in FIG. 2 givesrise to many advantages. In comparison to the simple single-curvecorners of the rib 1 illustrated by FIG. 1, the joggles 5 a, 5 b of therib of FIG. 2 i) provide extra flexibility at the corners, ii) lower thestrain in the material in the corner portion of the rib and iii)introduces compression through-thickness forces (in the region 12 ofnegative curvature), all three of which reduce the problems associatedwith delamination or the formation of cracks (and the propagationthereof) at the corners caused by the stresses imposed on the rib 1 by,for example, the fuel 2. Also, the shape of the joggles helps reduce theeffects of “spring-in” of the rib after manufacture.

FIG. 4 shows an aircraft 14 comprising a fuselage 13, wings 19,tail-plane 17 and fin 18. The wings 19, being conventional in structure,are formed of a front wing spar 20 and a rear wing spar 16, in theregion of the leading and trailing edges, respectively, of the wing 19.Attached to and between the spars 16, 20 are ribs 1. Each rib is of thegeneral form described with reference to the rib 1 shown in FIG. 2 (FIG.2 showing the rib in the cross-section taken along the line A-A of FIG.4). The invention is particularly, but not exclusively, applicable tolarger aircraft such as passenger carrying aircraft or freight carryingaircraft.

It will be appreciated that various modifications could be made to theabove-described embodiment without departing from the scope of thepresent invention. For example, the upper, middle and lower portions ofthe rib need not be planar in form and the rib could instead besinusoidal in shape along its length. Also, the joggles described abovecould be used to advantage in other structural parts of an aircraft,where such parts are required to have bends or corners. The presentinvention is of particular advantage in the case where the structuralpart is a load bearing structure, for example, a structure that, in use,is subjected to non-trivial loads. For example, the spars of a wing ofan aircraft might be made from a laminated composite materialincorporating joggles.

1. A laminated composite material structure for forming a part of anaircraft, the structure comprising first and second laminated portionsangled with respect to each other and a third laminated portion beingcontinuous with and interposed between the first and second laminatedportions, wherein the third laminated portion includes at least oneunsupported region of positive curvature and at least one unsupportedregion of negative curvature for increasing flexibility of said thirdlaminated portion.
 2. A structure according to claim 1, wherein at leastone of the regions of curvature has a cross-section in the form of anarc having a substantially constant radius of curvature.
 3. A structureaccording to claim 1, wherein the third portion includes a first regionof curvature positioned between second and third regions of curvature ofthe opposite sign to that of the first region of curvature.
 4. Astructure according to claim 1, wherein the first and second laminatedportions are transverse to each other.
 5. A structure according to claim1, wherein the direction at any point along the notional route describedby the third portion moving from the first portion to the second portionincludes a positive component in the direction from the boundary betweenthe first and third portions to the boundary between the third andsecond portions.
 6. A structure according to claim 1, wherein thestructure is a load bearing structure.
 7. A structure according to claim1, wherein the first portion of the structure forms at least a part ofthe portion of a rib for a wing of an aircraft, the portion of the ribbeing attachable to a wing skin of an aircraft.
 8. An aircraft wingstructure including a structure as claimed in claim
 1. 9. An aircraftincluding an aircraft wing structure as claimed in claim 8.